Sensor for measuring moisture and salinity

ABSTRACT

A sensor for measuring the moisture and salinity of a material is disclosed herein. The sensor preferably includes a soil moisture circuit, a soil salinity circuit and a probe structure. The soil moisture circuit includes a high frequency oscillator, a voltage meter and a reference capacitor. The soil salinity circuit includes a low frequency oscillator, a voltage meter and a reference resistor. A third voltage meter allows for voltage outputs to be measured to calculate soil moisture and soil salinity values.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 12/124,977, filed on May 21, 2008.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention related to a sensor for measuring the moisturecontent and salinity of a material. More specifically, the presentinvention relates to a sensor for measuring the moisture content andsalinity of soil.

2. Description of the Related Art

The prior art discusses several soil monitoring sensors.

There have been several sensor approaches to measuring the moisture andsalinity of soil. One such approach is described by U.S. Pat. No.5,479,104 (the '104 patent). The sensor of the '104 patent uses oneoscillator and one frequency, a bridge type scheme with two resistorsfunctioning as reference bridge elements, and three AC meters to measurethe unknown capacitance of the soil (C_(s)) and resistance of the soil(R_(s)), where the capacitance can be directly related to soil moistureand the resistance to salinity. The use of such a sensor requires thefollowing steps: powering the sensor (which turns on the oscillator);reading the three AC meters; performing numerous calculations todetermine C_(s) and R_(s); applying calibration equations to convertC_(s) to soil moisture measurements and R_(s) to salinity measurements;and turning off the sensor.

The difficulty with such a sensor scheme is that the calculations todetermine C_(s) and R_(s) are very complex relations of two ratios: (1)AC Meter 3/AC Meter 1 and (2) AC Meter 2/AC Meter 1. These calculationsare particularly complex in actual systems with higher order effectspresent, thus requiring the use of complex number equations.

A further difficulty is that AC meters that operate at such highfrequencies are difficult and expensive. This problem is often addressedby using simple diode detectors in place of standard AC meters. Diodedetectors, however, become highly non-linear when the AC voltage lowersto around 0.2 volts such that it becomes difficult to relate the DCmeter voltage reading to the actual AC voltage level being measured. Atsuch voltages, the detectors also become increasingly sensitive tovariations in the components used. Further, the diode detectors displaya significant temperature dependence that requires correction.

Referring to the '104 patent, the basic scheme is shown in FIG. 2 of the'104 patent. To begin with, in the '104 patent, one frequency is used,50 MHz in the built version of that sensor. It employs a bridge typescheme with R1 and R2 as known reference bridge elements, along withthree AC meters to measure the unknown capacitance and resistance of thesoil (element 24). The capacitance can be directly related to soilmoisture and the resistance to salinity. The calculations to determineCs and Rs are very complex (particularly in actual systems with higherorder effects present) relations of two ratios-(AC Meter 3/AC Meter 1)and (AC Meter 2/AC Meter 1). Essentially there are two measured ratioswhich are then used to solve for two unknowns-Cs, Rs. The simpler firstorder calculations are shown in the patent (column 6, near line 50).

The actual steps in the measurement process are as follows: power thesensor (which activates the oscillator), read the three AC meters,perform the calculations to obtain the Cs value to soil moisture and Rsvalue to salinity, and then deactivate the sensor.

Because AC meters that operate at these high frequencies are difficultand expensive, simple diode detectors are used as a proxy for AC meters.These are shown in FIG. 3 of the '104 patent. The output of these metersis a DC voltage read by the DC Meters. The DC meters are inexpensive,but have problems. Once the AC voltage gets low (around 0.2 volts orso), the detectors stop working well. Even at AC voltages around 0.3V,the detectors become highly non-linear (i.e. it is not easy to relatethe DC Meter to the actual AC voltage level being measured) and thedetectors become increasingly sensitive to component variations (thediode, resistor, and capacitor used). In addition, these detectors alsodisplay a significant temperature dependence that needs to be correctedfor in the measurement. At the end of the line, the calculations neededto correct for detector temperature effects and non-linearity, as wellas calculating Cs and Rs including the higher order effects, whichbecomes very complicated.

To provide accurate moisture and salinity measurements, therefore,current sensors must calculate capacitances and resistances, includinghigher order effects, using very complicated complex number equationsthat can also correct detector temperature effects and detectornon-linearity.

Thus, there is need for a sensor to measure the capacitance andresistance of soil that uses simpler calculations, provides improveddetector performance, and, ultimately, provides more accurate moistureand salinity measurements.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a sensor monitor and a method thereofto resolve the issues associated with the sensor monitors of the priorart.

One aspect of the present invention is a method of determining amoisture content value and a salinity value of a soil, the method beginswith providing a sensor. The sensor includes a probe conductingstructure to be placed in the soil to form a capacitor, a soil moisturecircuit and a soil salinity circuit. The soil moisture circuit includesa high frequency oscillator for applying a first electrical stimulus tothe probe structure, a reference capacitor connected in series to thehigh frequency oscillator, and a first voltage meter located between thehigh frequency oscillator and the reference capacitor. The soil salinitycircuit includes a low frequency oscillator for applying secondelectrical stimulus to the probe structure, a reference resistorconnected in series to the low frequency oscillator, and a secondvoltage meter located between the low frequency oscillator and thereference resistor. The low frequency oscillator is substantially lowerin frequency than the high frequency oscillator. The soil salinitycircuit and the soil moisture circuit are connected between thereference capacitor and the reference resistor, at which point the soilmoisture circuit and the soil salinity circuit are connected to theprobe structure and a third voltage meter.

The method also includes measuring voltages V1, V2, and V3, wherein V1is the output voltage measured by the first voltage meter when the lowfrequency oscillator and the high frequency oscillator are inactive, V2is the output voltage measured by the second voltage meter when the lowfrequency oscillator and the high frequency oscillator are inactive, andV3 is the output voltage measured by the third voltage meter when thelow frequency oscillator and the high frequency oscillator are inactive.The method also includes measuring voltages V2′ and V3′, wherein V2′ isthe output voltage measured by the second voltage meter when the lowfrequency oscillator is active and the high frequency oscillator isinactive, and V3′ is the output voltage measured by the third voltagemeter when the low frequency oscillator is active and the high frequencyoscillator is inactive. The method also includes measuring voltages V1′and V3″, wherein V1′ is the output voltage measured by the first voltagemeter when the low frequency oscillator is inactive and the highfrequency oscillator is active, and V3″ is the output voltage measuredby the third voltage meter when the low frequency oscillator is inactiveand the high frequency oscillator is active. The method also includescalculating a capacitance of the soil as a function of (V3″−V3)/(V1′−V1)to obtain a soil moisture content value. The method also includescalculating a resistance of the soil as a function of (V3′−V3)/V2′−V2)to obtain a soil salinity value.

The critical difference between the sensor of the present invention andthe sensor of the '104 patent lies in the fact that two separate bridgecircuits and frequencies are used and that the soil moisture leg uses acapacitor for a reference element while the soil conductivity leg uses aresistor.

There are a number of advantages. By using a capacitor as the bridgeelement in measuring the capacitance (soil moisture) and a resistor inthe bridge circuit for measuring soil conductivity (orresistance-related to salinity), the bridge circuit should be “tuned”appropriately. By using the appropriate bridge element in each leg,there are two benefits: 1) better resolution and accuracy in themeasurement of the unknown value; and 2) greatly simplified mathematicalalgorithms to calculate the desired quantities. Third, the AC meter ofthe present invention is vastly improved.

The measurement process is as follows (note that, while not shown, inthe sensor of the present invention there is a digital signal processor(a microcontroller) that can turn on and off the oscillators as well asmeasure the DC voltages produced by the detectors). First, bothoscillator 1 and 2 are turned off and the voltage of all the detectorsis measured (referred to as V1, V2, and V3 for AC meters 1 through 3respectively). The low frequency oscillator 2 is turned on and thedetector voltages for AC meter 3 and 2 are measured (V3′ and V2′).Oscillator 2 is then turned off, and the high frequency oscillator 1 isturned on and the voltages of AC meter 1 and 2 are measured (V1′ andV2′). It can then be shown that, due to the careful design of thecircuit, that:Cs=f(Ratio HF), Rs=g(Ratio LF)

Where: Ratio HF=(V2″−V2)/(V1′−V1) Ratio LF=(V2′−v2)/(V3′−V3)

In general these functions are simple polynomials that include all thehigher order effects. This data reduction is much simpler than in thesensor of the '104 patent. Also note that the measurement of capacitanceis not affected by the resistance present within the soil (withintypical ranges) and similarly, the resistance is not affected by thecapacitance present. This is achieved by the “tuned” bridge circuit andcareful choice of component values.

Returning to the AC meter embodiment used in the sensor of the presentinvention, it should be noted that the biasing that has been addeddramatically improves the detector performance. The presence of the biasvoltage allows the detector output to be measured in the absence of anyAC signal being present in the circuit. This allows the sensors to be“tared” and captures the variation in components in the detector andtemperature drifts. It is not possible to do this with the sensor of the'104 patent, the diodes must be carefully matched (i.e. have the samecritical properties which isn't easy and can be a manufacturingnightmare) and complicated and somewhat uncertain temperaturecorrections applied. In addition, the biased detectors have theadvantage of being able to respond to very small AC signals and are muchmore accurate at low AC signal levels. Since the individual detectorsare tared, they don't require carefully matched parts as well makingmanufacturing easier.

The invention overcomes difficulties encountered in the prior art byusing two separate bridge circuits and frequencies to measure themoisture and salinity of soil, where a soil moisture bridge circuit usesa capacitor for a reference element and a soil salinity bridge circuituses a resistor for a reference element.

Having briefly described the present invention, the above and furtherobjects, features and advantages thereof will be recognized by thoseskilled in the pertinent art from the following detailed description ofthe invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a preferred embodiment of the circuitry of the sensorof the present invention.

FIG. 1A is a block diagram of a sensor of the present invention.

FIG. 2 illustrates an embodiment of a biased diode detector used by theinvention as an AC meter.

FIG. 3 is a perspective view of a sensor apparatus of the presentinvention.

FIG. 4 is a table of experimental results on sand showing measuredvalues in a drying experiment with water percentage as expressedpercentage by volume of water in a sand sample.

FIG. 5 is a graph of a calibration curve illustrating water content as afunction of RP.

FIG. 6 is a table of experiments in aqueous saline solutions of knownconductivity ranging from 0.1 to 4.96 dS/m and replicated for fourdifferent sensors.

FIG. 7 is a continuation of the Table in FIG. 6.

FIG. 8 is a graph of a calibration curve showing variation in 1/RC withincreasing sample conductivity.

FIG. 9 is a schematic of an embodiment of a sensor apparatus of thepresent invention.

FIG. 10 is a circuit schematic of an embodiment of the sensor apparatusof the present invention.

FIG. 10A is an isolated circuit diagram of a preferred voltage meter ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a circuit diagram of a preferred embodiment of thecircuit 40 of the sensor apparatus 20 of the present invention. FIG. 2illustrates a detailed circuit diagram of a preferred section of thecircuit 40. FIG. 1A is a block diagram of the sensor apparatus 20. Asshown in FIG. 1A, the sensor apparatus 20 preferable includes a digitalsignal processor 35 connected to a moisture circuit 22 and a salinitycircuit 23, which are both connected to a probe structure 21.

The probe structure 21 is placed in the soil which is to be measured.The probe structure 21 forms an effective coaxial capacitor within thesoil. Such probe structures are well known in the art, and typicallyinclude a base and elongated conductors extending from the base anddisposed around a central elongated conductor. The digital signalprocessor 35 or microprocessor, facilitates the process, allowing formultiple conducting structures to be inserted into the soil (or othermedia of interest) as well as cabling to provide power and transfermeasurement results to recording or control instrumentation. The probestructure 21, which when placed in soil forms, electrically, the circuitelements Cs 21 a and R_(s) 21 b as shown in FIG. 1, and are referred toas forming a “capacitor.” The probe structure 21 can be arranged in avariety of different geometries many of which are shown in U.S. Pat.Nos. 2,870,404, 4,288,742, and 4,540,936, all of which are herebyincorporated by reference in their entireties. The conducting structuresof the afore-mentioned '104 patent can also be included in the presentinvention a probe structure 21. The probe structure 21 can be made ofmetal, printed circuit board, or other electrically conductivematerials. Depending on the media of interest, the range of expectedC_(S) and R_(s) to be measured and frequencies employed, many differentgeometries and sizes can be employed as the probe structure 21 insensor.

FIG. 1 shows an embodiment of the circuit 40 of the sensor 20 of thepresent invention. As illustrated in FIG. 1A, the sensor 20 uses twoseparate bridge circuits and frequencies that coexist together withoutsignificantly perturbing each other. The soil moisture circuit 22preferably includes, as shown in FIG. 1, a high frequency oscillator 24,a first voltage meter 26, and a reference capacitor C_(R) 29 of a knownvalue. The soil salinity circuit 23 preferably includes, as shown inFIG. 1, a low frequency oscillator 25, a third voltage meter 28, and areference resistor R_(R) 30 of a known value. The circuits 22 and 23share a second voltage meter 27, as well as the probe structure 21positioned in the soil, the electrical equivalent of which isrepresented by the lower circuitry of FIG. 1 and designated C_(S) 21 aand R_(s) 21 b. In this embodiment of the invention, each of the firstvoltage meter 26, the second voltage meter 27 and the third voltagemeter 28 is preferably an alternating current (“AC”) voltage meter. Asshown in FIG. 2, on one end a diode 80 is connected to a resistor RD 83and capacitor C_(D) 84, as well the voltage meter 26. On another end,the diode 80 is connected to a bias resistor 81 with a biased outputvoltage 82.

The measurement process is as follows. First, the oscillators 24 and 25are deactivated and voltages V1, V2, and V3 are measured, where V1, V2,and V3 are the voltages at the first voltage meter 26, the secondvoltage meter 27 and the third voltage meter 28, respectively, when theoscillators 24 and 25 are deactivated. Next, the low frequencyoscillator 25 is activated and voltages V2′ and V3′ are measured, wherevoltages V2′ and V3′ are the voltages at the second voltage meter 27 andthe third voltage meter 28, respectively, when only the low frequencyoscillator 25 is activated. Next, the low frequency oscillator 25 isdeactivated, and the high frequency oscillator 24 is activated, andvoltages V1′ and V3″ are measured, where voltages V1′ and V3″ are thevoltages at the first voltage meter 26 and the third voltage meter 28,respectively, when only the high frequency oscillator 24 is activated.

The capacitance of the soil is a function of the ratio of the highfrequency (“HF”) voltage measurements, and the resistance of the soil isa function of the ratio of the low frequency (“LF”) voltagemeasurements:

The following equations demonstrate the afore-mentioned:C _(s) =f(Ratio HF)R _(s) =f(Ratio LF)

Where: Ratio HF=(V3″−V3)/(V1′−V1)

and Ratio LF=(V3′−V3)/(V2′−V2)

Note that, while not shown, a person of ordinary skill would understandto use devices such as a digital signal processor 35 or amicrocontroller to activate and deactivate the oscillators 24 and 25, aswell as measure the voltages produced by the first voltage meter 26, thesecond voltage meter 27 and the third voltage meter 28.

The present invention provides multiple advantages over the prior art.For example, when attempting to measure soil moisture, the presentinvention uses a high frequency oscillator 24, but when measuring soilsalinity the present invention uses a low frequency oscillator 25.

To accommodate both measurements, other sensors use a compromise middlefrequency. Such a compromise, however, limits the accuracy and range ofmeasurement of the sensor. The dual circuits 22 and 23 of the presentinvention avoid this compromise by enabling the application of both highand low frequencies. Second, by using a capacitor as the bridge elementin measuring the soil's capacitance (C_(s)) and a resistor as the bridgeelement in measuring soil's resistance (R_(s)), the bridge circuit is“tuned” appropriately in that the soil capacitance can be determinedwith the least error introduced by the soil resistance, and conversely,the soil resistance can be determined with the least error introduced bythe soil capacitance. That is, the bridge elements provide that theC_(s) and R_(s) can be compared to a comparable C_(R) and R_(R).

By using the appropriate bridge element in each circuit, the inventionprovides better resolution and accuracy in the measurement of theunknown values and greatly simplified mathematical algorithms tocalculate the desired quantities. Higher order effects in practicalsensors such as stray capacitance, can be handled using relativelysimple polynomials. The simplicity of this sensor, as opposed forexample to the embodiment described in the '104 patent, arises from the“tuned” nature of the circuits which allow soil capacitance andresistance effects to be separately determined without any mutualinteraction. Thus, the functions described above are simple polynomialsthat include all the higher order effects, and therefore enablesignificantly simpler data reduction.

Further, the tuned bridge circuit and a careful choice of componentvalues ensures that the measurement of capacitance does not affect theresistance present within the soil (within typical ranges) and,similarly, the measurement of resistance does not affect the capacitancepresent.

FIG. 2 illustrates an embodiment of a biased diode voltage meter used bythe invention as an AC voltage meter. The diode voltage meters measure apeak voltage present in the circuit 40 (subject to some correction andtemperature compensation) and are related to the root mean square (RMS)voltage present in the case of sinusoidal voltages. Standard RMS ACmeters that operate at the frequencies necessary for soil moisture andsalinity measurements are typically bulky, require extensivecalibration, expensive, and electrically invasive in that their presencegreatly perturbs the desired measurement through stray capacitance andother effects.

Thus, the invention preferably uses diode voltage meters in their place.Diode voltage meters provide an output that is a direct current (“DC”)voltage read by a DC voltage meter. The diode voltage meters aretypically inexpensive and work well at high frequencies. Those ofordinary skill in the art are familiar with using diode detectors as ACvoltage meters. In one embodiment of the sensor 20, diode voltage metersare used as they produce an easily measured DC output voltage, areinexpensive, work well at both high and low frequencies, and do notsignificantly perturb the measurement.

The diode voltage meter utilized in one embodiment of the presentinvention is unique because it is biased, adding a bias voltage V_(B) 82and bias resistor R_(B) 81 to the standard diode voltage meter circuit,as shown in FIG. 2. This biasing dramatically improves the voltage meterperformance by improving the linearity of the sensor 20, allowing forsmaller AC voltages to be measured, and by removing the effects ofvariations in individual diodes and temperature effects. The presence ofthe bias voltage allows the output of the voltage meter 26, 27 or 28 tobe measured in the absence of any AC signal being present in the circuit40. The bias voltage, which is present at all times when the sensor 20is powered, allows for the output of the voltage meter 26, 27 or 28 tobe measured with no AC signal present, and the effects of temperaturedrift and variation in individual diode drop are determined by the fixedoutput level.

When the output of the diode voltage meter, is measured again, this timewith an AC signal present, the difference in the outputs of the voltagemeters 26, 27 and/or 28 essentially represents the AC signal valuepresent without any errors introduced by variations in diode drop andtemperature. Because of this difference approach, diodes do not need tobe carefully characterized and matched within the circuit 40, therebysimplifying sensor manufacturing and reducing cost.

In addition, to obtain a separate temperature measurement, correctionalgorithms do not need to be used to account for temperature effects.Diode voltage meters in the absence of biasing do not allow for themeasurement of AC signals much below 0.2V due to the voltage drop acrossthe diode. The presence of the bias voltage allows AC voltages down toaround 0.01V to be accurately measured which allows the sensor 20 torespond to a larger variation in C_(S) and R_(s). This allows the sensor20 to be “tared” to capture the variation in components in the voltagemeter and temperature drifts. The bias voltage also enables the voltagemeters 26, 27 and 28 to respond to very small AC signals, and providemuch more accurate measurements at low AC signal levels. Further, sincethe voltage meters 26, 27 and 28 are preferably tared, the individualvoltage meters do not require carefully matched parts, thus makingmanufacturing easier.

FIG. 3 illustrates a preferred embodiment of a sensor apparatus 20 ofthe present invention. The sensor apparatus 20 is preferablypre-calibrated for typical sand, silt, or clay soils. The sensorapparatus 20 may need to be adjusted for other soils due to a possibledifference in composition and placement. The sensor apparatus 20 hasexcellent accuracy and reliability. The sensor apparatus 20 canpreferably provide a reading of +/−1.0% of a moisture reading from 0 tosaturate at <5 dS/m conductivity and +/−3.0% of a moisture reading from0 to saturation at >5 dS-10 dS/m conductivity. As for salinity, thesensor apparatus 20 of the present invention has a +/−2% of conductivityreading up to 5 dS/m (5 dS/m=30% seawater). The temperature reading is+/−1 degree Celsius from −10 degrees Celsius to +50 degrees Celsius. Themeasurement repeatability is preferably <<1%. The sensor apparatus 20preferably includes a housing 50, a probe structure 21 and a cable 51for power and communications with a control center. As shown in FIG. 9,the housing 50 preferably contains the circuit 40 and digital signalprocessor 35. The probe structure 21 extends from the housing 50, andcable 51 allows for connections to the circuit 40 and digital signalprocessor 35.

The present invention may be utilized with the system and method ofGlancy, et al., U.S. Patent Publication Number 2006/0178847, publishedon Aug. 10, 2006, for an Apparatus And Method For Wireless Real TimeMeasurement And Control Of Soil And Turf Conditions (U.S. patentapplication Ser. No. 11/350,328 filed on Feb. 8, 2006), which is herebyincorporated by reference in its entirety.

The present invention provides a sensor apparatus 20 having a circuit 40that allows for measuring the electrical conductivity and dielectricpermittivity of a wide range of media. Specifically, these media includesoils, water, grain, brackish waters, ices, and the like media.

As shown in FIG. 9, the housing 50 protects the circuitry from moistureand other hazards. The circuit 40 produces analog signals that are usedto determine dielectric permittivity (and temperature if desired). Adata reduction circuitry is optionally included and includes amicrocontroller to measure the analog voltages and perform thecalculations to determine the electrical properties and relate them ifdesired to other properties (such as soil moisture and soil salinity). Aprobe structure 21 preferably includes electrically conducting surfacesthat can be inserted into the media of interest. The cable 51 allows forthe calculated outputs to be sent to other instruments Alternatively,wireless or other communication protocols can be used without departingfrom the scope and spirit of the present invention.

The housing 51 is preferably composed of stainless steel or PVC.However, those skilled in the pertinent art will recognize that thehousing 51 may be composed of other materials without departing from thescope and spirit of the present invention.

The circuit 40 converts the media's dielectric permittivity andconductivity to analog voltage signals that are used to calculate theelectrical properties. These calculations are performed using thedigital signal processor 35 in the sensor 20, or alternatively theanalog voltage signals are transferred to an external instrument for therequired calculations.

The digital signal processor 35 is used to measure the raw analogsignals, convert them to a digital format, calculate electricalproperties, calculate related properties (such as soil moisture andsalinity) as well as control the oscillators (discussed below). Thedigital signal processor 35 can be used to output the calculatedparameter digitally or in an analog format over the cable 51, oralternatively through the use of additional circuitry, wirelessly.

The probe structure 21 connects electrically to the circuit 40 andpreferably includes at least two electrically conducting surfaces orstructures. The probe structure 21 is inserted into the media to form acapacitor. The probe structure 21 is preferably composed of metal,printed circuit board material (with copper layer), or other materials.

The cable 51 allows power to be supplied externally as well as allowingfor the output of the analog signals of the circuit 40. In an embodimentwith the digital signal processor 35, the oscillators 24 and 25 aredirectly controlled and the cable 51 can be used to transmit thecalculated electrical properties.

As shown in FIG. 10, the sensing electronic circuitry 40 converts thesoil dielectric permittivity and conductivity into analog signals asdescribed below.

The circuit 40 preferably employs two oscillators, a high frequencyoscillator 24 (125 MHz for example) and a low frequency oscillator 25(10 MHz for example) powered by +V (a DC voltage), and which can becontrolled (turned off and on) by a capacitor C_(HI) and a capacitorC_(LO), respectively. Those skilled in the pertinent art will recognizethat oscillators with a wide range of operating frequencies can be usedwith the present invention without departing from the scope and spiritof the present invention. The oscillating electrical outputs appear at“OUT.” A filter 57, which is connected to high frequency oscillator 24is preferably a bandpass filter centered at the primary frequency of thehigh oscillator 24 and removes higher order harmonics to preferablyproduce a nearly sinusoidal output. Capacitor C2 69 and resistor R4 70are connected to the low frequency oscillator 25 to form a low passfilter 55 (designated by dotted lines) which removes any higher orderharmonics, specifically if the low oscillator 25 is a square waveoscillator.

A plurality of voltage meters 26, 27 and 28 are used to measure the AClevel present at various points 26 a, 27 a and 28 a in the circuit 40.As shown in FIG. 10A, each of plurality of meters 26, 27 and 28preferably comprises a diode 93 connected with a capacitor 95 andresistor 97 in parallel (FIG. 10A only shows a voltage meter 26). Theoutput “O” of each of the plurality of meters 26, 27 and 28 ispreferably a direct current (“DC”) voltage (measured between the diodeand the capacitor and resistor) and is indicative of the amplitude ofthe AC stimulus present at the point 26 a, 27 a and 28 a in the circuit40 to which the other end of the diode 93 is connected.

A biasing network is formed by resistor R1 62, resistor R2 64, resistorR3 65, resistor R5 67 and capacitor C1 66. The biasing network ensuresthat even in the absence of any AC stimulus in the circuit 40, each ofthe voltage meters 26, 27 and 28 has a DC voltage present, thusproviding a non-zero output for each of the voltage meters 26, 27 and28. In addition, resistor R2 64 is also a conductivity referenceelement, which is described in greater detail below.

Two capacitors CB 61 and 68 ensure that the bias voltages are drainedthrough the filter 57 as well as preventing any DC voltages from beingpresent on the probe structure 21, which can result in deleteriousgalvanic reactions and electrode polarization. Capacitor CB 61 isselected to have a sufficiently high capacitance to have a negligibleimpedance at both the high and low frequencies employed so as tonegligibly affect the measurements.

A capacitor CR 63 is preferably a precision capacitor used to form areference element for the dielectric permittivity measurements.

The sensor 20 preferably has two main paths through which the electricalmeasurements are made. First, the high frequency path of the soilmoisture circuit 22 (used to measure the dielectric permittivity) isthrough the high frequency oscillator 24, the filter 57, the capacitorCB 61, by the first voltage meter 26, the capacitor CR 63, by the secondvoltage meter 27, the capacitor CB 68 and into the probe structure 21.Second, the low frequency path, or the soil salinity circuit 23, isthrough low frequency oscillator 25, the resistor R4 70, the capacitorC2 69, by the third voltage meter 28, resistor R2 64, by the secondvoltage meter 27, capacitor CB 68 and into the probe structure 21. Byjudicious choice of the elements of the circuit 40, the high and lowfrequency paths can share many elements of the circuit 40 as well as theprobe structure 21, while having very little effect on the otherpathway.

A preferred measurement can be made in the following manner and thetables of FIGS. 4 and 6 list the measurement values. Those skilled inthe pertinent art will recognize that different procedures are possiblewithout departing from the scope and spirit of the present invention.First, with both the high frequency oscillator 24 and the low frequencyoscillator 25 deactivated, the outputs of the first voltage meter 26,the second voltage meter 27, and the third voltage meter 28 aremeasured, at points 26 a, 27 a and 28 a respectively, and referred torespectively as O1, O2, O3. The high frequency oscillator 24 is thenactivated and the first voltage meter 26 and the second voltage meter 27are measured, at points 26 a and 27 a respectively, and referred torespectively as O1′ and O2′. The high frequency oscillator 24 is thendeactivated and the low frequency oscillator 25 is activated and thesecond voltage meter 27 and the third voltage meter 28 are measured, atpoints 27 a and 28 a respectively, and referred to respectively as O2″and O3″. In the case where an oscillator 24 or 25 is deactivated, thevoltage meter output is indicative of a “baseline” voltage meter output.When an oscillator 24 or 25 is activated, the voltage meter isindicative of the baseline level plus the AC stimulus amplitude presentat the measurement point 26 a, 27 a and/or 28 a.

The measured voltages can then be used to calculate the followingratios: RP=(O2′−O2)/(O1′−O1) and RC=(O2″−O2)/(O3″−O3).

The probe structure 21 forms a capacitance C_(s) and conductivity CONDSdetermined by the dielectric permittivity and electrical conductivity ofthe sample the conducting structure is placed in as well as geometricalfactors (spacing, size, geometry, etc. of conducting surfaces). The twocircuit pathways form bridge circuits (C_(R) and C_(s) on the highfrequency path) and (R2 and CONDS on the low frequency path). To thefirst order it can be shown that:C _(s)/(C _(s) +CR)=RP and CONDS/(CONDS+R2)=RC

This allows one to solve for the dielectric permittivity of the sampleand its conductivity when the scaling factor introduced by thegeometrical factors of the sensing structure are determined, as CR andR2 are known values. Furthermore, these calculations can be refined toremove high order effects (finite electrical length, pathway coupling,etc.) through calibration with known samples.

Data reduction circuitry can be used to automate this procedure as wellas perform the required calculations. A suitable data reduction circuithas been developed and tested.

The circuit 40 contains many novel features that greatly improveaccuracy, cost, and practicality. They include novel biased detectors, anovel biasing scheme, a novel differential measurement scheme, a noveldual frequency scheme, novel matched reference elements, and novel powersupply variation rejection.

In a preferred embodiment, the diode based voltage meters 26, 27 and 28are always forward biased regardless of the signal level. Depending ondiode type, at forward drops of less than about 0.3V or so, little or nocurrent flows. Small AC signals with amplitudes less than 0.3V inamplitude would be undetectable without the biasing employed.

The circuitry has been designed to allow the biasing to be provided tothe voltage meters 26, 27 and 28 while simultaneously allowing anaffected measurement to be made. This is accomplished through the use ofblocking capacitors and selected values for the biasing circuitry.

Because the outputs of the voltage meters 26, 27 and 28 are measuredwith and without AC stimulus being present non-matching diodes 85 areused, as the ratios RP and RC are essentially independent of theindividual diode drops. Furthermore, the temperature drift of the diodesis automatically nulled-out by this procedure.

The present invention measures conductivity at low frequencies so thatcapacitive effects do not distort the measurement and dielectricpermittivity at high frequencies so that conductive effects do notdistort the measurement. This design allows the use of two frequenciesin one sensor with the same probe structure 21 allowing for improvedaccuracy in a wide range of media.

The bridge element in the dielectric permittivity measurement is acapacitor, CR. It can be shown that using a capacitor as a referenceelement offers superior rejection of distortion due to conductivityeffects. Likewise, the bridge element in the conductivity measurement isa resistor which offers superior rejection of distortion due tocapacitive effects.

As the measurements is “ratio metric”, the output level of oscillators24 and 25 is essentially linear with power supply voltage, and thebaseline voltage meter level is measured (and accounted for in thecalculations) the calculations of electrical parameters is highlyinsensitive to variations in power supply voltage over a significantrange allowing for a low cost, simple power supply to be used.

As shown in FIG. 4, a table 200 lists values for voltage metermeasurements O1, O2, O3, O1′, O2′, O2″ and O3″. Also listed are valuesfor the calculations RP and RC as well as a percentage of moisture(water %) in the media. FIG. 5 illustrates a graph 202 with acalibration curve of the water % (Y axis) as a function of RP (X axis).RP is used to calculate capacitance and is correlated to soil moisture.

As shown in FIG. 6, a table 300 lists values for voltage metermeasurements O2, O3, O2″ and O3″. Table 300 also lists values for RC andthe type of media sensors for four different sensors. The type of mediais aqueous saline solutions of known conductivity spanning 0.1 to 4.96dS/m. Each sensor set is for a different type of media. Each sensor setincludes values for each of the four different sensors designated 3, 5,8 and 11 in the table 300. As shown in FIG. 7, a table 301 lists theaverage RC value and the standard deviation value from the values intable 300 of FIG. 6. FIG. 8 illustrates a graph 302 showing acalibration curve of the variation in 1/RC (X axis) with increasingsample conductivity (Y axis).

From the foregoing it is believed that those skilled in the pertinentart will recognize the meritorious advancement of this invention andwill readily understand that while the present invention has beendescribed in association with a preferred embodiment thereof, and otherembodiments illustrated in the accompanying drawings, numerous changesmodification and substitutions of equivalents may be made thereinwithout departing from the spirit and scope of this invention which isintended to be unlimited by the foregoing except as may appear in thefollowing appended claim. Therefore, the embodiments of the invention inwhich an exclusive property or privilege is claimed are defined in thefollowing appended claims.

1. A device for determining a moisture content value and a salinityvalue of a soil, the device comprising: a probe conducting structure tobe placed in the soil to form a capacitor; a soil moisture circuitcomprising a high frequency oscillator for applying a first electricalstimulus to the probe structure, a reference capacitor connected inseries to the high frequency oscillator, and a first voltage meterlocated between the high frequency oscillator and the referencecapacitor; a soil salinity circuit comprising a low frequency oscillatorfor applying second electrical stimulus to the probe structure, areference resistor connected in series to the low frequency oscillator,and a second voltage meter located between the low frequency oscillatorand the reference resistor, wherein the low frequency oscillator issubstantially lower in frequency than the high frequency oscillator; athird voltage meter; and a processor, the processor calculating acapacitance of the soil as a function of (V3″−V3)/(V1′−V1) to obtain asoil moisture content value wherein V1 is the output voltage measured bythe first voltage meter when the low frequency oscillator and the highfrequency oscillator are inactive, V3 is the output voltage measured bythe third voltage meter when the low frequency oscillator and the highfrequency oscillator are inactive, V1′ is the output voltage measured bythe first voltage meter when the low frequency oscillator is inactiveand the high frequency oscillator is active, and V3″ is the outputvoltage measured by the third voltage meter when the low frequencyoscillator is inactive and the high frequency oscillator is active, andthe processor calculating a resistance of the soil as a function of(V3′−V3)/V2′−V2) to obtain a soil salinity value wherein V2 is theoutput voltage measured by the second voltage meter when the lowfrequency oscillator and the high frequency oscillator are inactive, V2′is the output voltage measured by the second voltage meter when the lowfrequency oscillator is active and the high frequency oscillator isinactive, and V3′ is the output voltage measured by the third voltagemeter when the low frequency oscillator is active and the high frequencyoscillator is inactive.